John M. Dawson

The use of vancomycin and tobramycin in acrylic bone cement: biomechanical effects and elution kinetics for use in joint arthroplasty.

We examined the effects of vancomycin on the compressive strength and fatigue life of bone cement and the pharmacokinetics and antimicrobial activity against methicillin-resistant Staphylococcus aureus of vancomycin eluted from bone cement, both alone and in combination with tobramycin. Two cements, Palacos and Simplex, were tested. Three antibiotic preparations were tested: lyophilized vancomycin (vancomycin-L), vancomycin powder (vancomycin-P), and tobramycin powder (Lilly, Indianapolis, IN). Although antibiotics did not significantly affect compressive strength, the fatigue life of bone cement was significantly decreased with vancomycin. Thus, fatigue testing revealed effects on cement strength not apparent by compression testing. Vancomycin-P had a substantially less detrimental effect on fatigue strength than vancomycin-L. Vancomycin-P elutes less efficiently than tobramycin. Although relatively little vancomycin-P eluted from bone cement, it retained biologic activity.

Biomechanical analysis of three surgical approaches for lumbar burst fractures using short-segment instrumentation

Burst fractures of the lumbar spine that are located below the thoracolumbar junction present a challenge when operative management is indicated. Short-segment instrumentation offers the advantage of incorporating fewer motion segments in the fusion, but may not provide adequate long-term stabilization. The goal of this study was to assess the axial stiffness and torsional rigidity of several short-segment instrumentation procedures. Compressive axial stiffness and torsional rigidity were measured in six intact porcine lumbar spines (L1-L5). A corpectomy was performed to simulate a burst fracture injury and decompression. Posterior instrumentation, posterior instrumentation with an anterior strut (a wood block), and anterior instrumentation with an anterior strut one level above and one level below the fracture site were applied as treatment strategies. VSP plates (Acromed, Cleveland, OH) for posterior instrumentation and the Kaneda system (Acromed, Cleveland, OH) for anterior instrumentation were used. Load-displacement and torque-angle plots were generated and used to calculate 144 estimates of axial stiffness and 144 estimates of torsional rigidity for these constructs. These analyses showed that, in comparison with the intact spine, posterior instrumentation alone was an average of 76% less stiff axially, posterior instrumentation with an anterior strut was 3% more stiff (not significantly different from intact), and anterior instrumentation with an anterior strut was 15% more stiff. Posterior instrumentation alone was an average of 30% less rigid in torsion, posterior instrumentation with an anterior strut was 26% less rigid, and anterior instrumentation with an anterior strut was 24% less rigid than the intact spine.(ABSTRACT TRUNCATED AT 250 WORDS)

Collagen expression and biomechanical response to human recombinant transforming growth factor beta (rhTGF‐β2) in the healing rabbit MCL

We investigated biomechanical and collagen expression in a healing bilateral rabbit medial collateral ligament (MCL) model to human recombinant transforming growth factor beta (rhTGF‐β2) at three and six weeks. Each rabbit had rhTGF‐β2 in a bioabsorbable pellet administered in one side, with the contralateral side serving as control (no rhTGF‐β2). All MCL healed with rhTGF‐β2 producing a profoundly increased scar mass at three weeks which decreased in size toward control at six weeks. In‐situ hybridization demonstrated collagen expression (type I and III) no different than control at three weeks, but by six weeks elevated expression of type I was seen. Biomechanical analysis at three weeks showed no effect of rhTGF‐β2 on structural properties. However, at six weeks rhTGF‐β2 significantly inhibited both the maximum load (p < 0.05) and energy absorbed (p < 0.05) with no change in stiffness. Despite increased type I collagen expression and profound increase in early scar mass, rhTGF‐β2 did not improve the structural properties. Whether the dose or mode of delivery is responsible for decline in structural properties cannot be determined in this design. We hypothesize investigations of healing ligaments to cytokines should have biologic and biomechanical properties correlated in the same study at a minimum of two time points.

The biomechanics of hybrid external fixation

OBJECTIVE The objective of this study was to ascertain which factors determine the rigidity of a hybrid external fixation frame in a tibial periarticular fracture model. DESIGN Laboratory investigation using a polyvinylchloride pipe periarticular tibia fracture model. SETTING Simulated periarticular tibia fractures were created in a tibia model. Instrumented specimens were tested in a biomaterials testing system. INTERVENTION Groups of simulated periarticular tibia fractures were stabilized with one of nine different external fixation constructs with components from one manufacturer. MAIN OUTCOME MEASUREMENTS Elastic stiffness was measured for each specimen in compression, torsion, flexion bending, extension bending, and varus and valgus bending. RESULTS The four-ring Ilizarov fixator was the stiffest in all modes of testing. There was a trend toward increasing stiffness with an increasing number of rings. Fixators constructed with multiple levels of fixation in the periarticular fragment were stiffer in all modes of testing. The additional spacing between wires gained by the use of a single thick carbon ring or the use of a drop wire three centimeters from a single ring did not increase stiffness in this model. Frame modifications intended to augment the bar-to-ring connection did not increase stiffness. The use of rings mounted with half-pins instead of a unilateral bar mounted with half-pins for diaphyseal fixation increased the stiffness of the frame only in torsional testing. CONCLUSIONS Although the ideal stiffness of an external fixator is unknown, our results show that the addition of more than one level of fixation in the periarticular fragment increases the stiffness of hybrid external fixators in this periarticular tibia fracture model. Augmentation of the ring-to-bar connection did not significantly affect the stiffness of the frame.

An analysis of the posterior epidural ligament role on the cervical spinal cord

Study Design Laminectomy was performed on cats to destroy the posterior epidural ligament. Evoked potentials and spinal cord blood flows quantified the spinal cord function before and after cervical flexion. Objectives This work describes a relationship between the loss of the posterior epidural ligaments and cervical spinal cord injury. Summary of Background Data The posterior epidural ligaments of the human cervical spine have been recently described. These ligaments theoretically prevent injury to the spinal cord by resisting collapse of the dura during cervical flexion. Methods The animals were divided into three experimental groups: 1) control: no laminectomy and standard position, 2) flexion control: no laminectomy and known imposed flexion, 3) laminectomy (C3‐C7) and flexion. Motor‐evoked potentials and evoked spinal cord potentials were recorded to quantify the spinal cord functions. Radioactive microspheres were used to quantify ischemia in the spinal cord. Results Control subjects showed blood flows of 36 mL/100 g/min (C3‐C4) to 46 mL/100 g/min (C7‐C8). Flexion control subjects did not experience significant reductions in blood flows or substantial changes in evoked potentials. The laminectomy plus flexion group experienced reduced blood flows and substantial motor‐evoked potentials and slight evoked spinal cord potential changes with 50°, 60°, and 70° flexion. Blood flow reduction was greater in the anterior half of the C7‐C8 segments compared with the posterior half at 60° flexion. Evoked spinal cord potentials were less vulnerable than motor‐evoked potentials. Conclusion The role of the posterior cervical epidural ligaments is to anchor the posterior dura mater to the ligamentum flavum. Loss of the ligaments allows anterior displacement of the posterior dura mater in flexion. Abnormal distribution of or lack of the cervical posterior epidural ligaments may lead to flexion myelopathy.

Host bone response to polyetheretherketone versus porous tantalum implants for cervical spinal fusion in a goat model.

Study Design. In vivo assessment of polyetheretherketone (PEEK) and porous tantalum (TM) cervical interbody fusion devices in a goat model. Objective. Directly compare host bone response to PEEK and TM devices used for cervical interbody fusion. Summary of Background Data. PEEK devices are widely used for anterior cervical discectomy and fusion but are nonporous and have limited surface area for bone attachment. Methods. Twenty-five goats underwent single-level anterior cervical discectomy and fusion and were alternately implanted with TM (n = 13) or PEEK devices (n = 12) for 6, 12, and 26 weeks. Both devices contained a center graft hole (GH), filled with autograft bone from the animals own iliac crest. The percentage of bone tissue around the implant, percentage of the implant surface in direct apposition with the host bone, and evidence of bone bridging through the implant GH were assessed by using backscattered electron imaging. Bone matrix mineral apposition rate was determined through fluorochrome double labeling, and sections were stained for histological analysis. Results. The TM-implanted animals had significantly greater volumes of bone tissue at the implant interface than the PEEK animals at all-time points. The TM animals also had a significantly greater average mineral apposition rate in the GH region at 6 and 12 weeks than the PEEK animals. No difference was observed at 26 weeks. A greater number of TM-implanted animals demonstrated connection between the autograft bone and both vertebrae compared with the PEEK implants. Histological staining also showed that the TM devices elicited improved host bone attachment over the PEEK implants. Conclusion. The TM implants supported bone growth into and around the implant margins better than the PEEK devices. TMs open cell porous structure facilitated host bone ingrowth and bone bridging through the device, which could be beneficial for long-term mechanical attachment and support in clinical applications.

Comparative biomechanics of hybrid external fixation

OBJECTIVE To compare the elastic stiffness, in several loading modes, of commercially available hybrid external fixation systems. DESIGN Laboratory investigation using a polyvinylchloride pipe periarticular tibia fracture model. SETTING Simulated periarticular fractures were created in an in vitro tibia fracture model. Instrumented specimens and intact controls were elastically tested in a biomaterials testing system. INTERVENTION Groups of simulated periarticular tibia fractures were stabilized with one of six different hybrid external fixator designs. MAIN OUTCOME MEASUREMENTS Elastic stiffness of each specimen was measured in compression, torsion, flexion bending, extension bending, and varus/valgus bending. RESULTS Fixators with multiple levels of fixation in the periarticular fragment, regardless of design, were stiffer than those with one level. Specifically, the EBI Ring Connector fixator was stiffer than all others in all modes of testing. The Ace, Synthes, Smith & Nephew Richards, and How medica fixators were mechanically similar. The Zimmer Torus fixator was the least stiff fixator tested. CONCLUSIONS Fixators with multiple levels of fixation in the periarticular fragment, regardless of design, were stiffer than those with one level. The choice of which hybrid external fixator to use should be made based not only on stiffness but also on ease of clinical application, patient comfort, customer support from the manufacturer, and cost. Clinical investigation of the efficacy of each of these devices is warranted.

isolated Muscle Atrophy of the Distal Upper Extremity in Cervical Spinal Cord Compressive Disorders

The purpose of this article is to clarify one mechanism of muscular atrophy of the distal upper extremity that we attribute to cervical compression (Hirayama-type amyotrophy) by medical imaging and anatomical studies of the cervical spine. Five young male patients (mean age: 22.6 years) with this disorder showed characteristic findings to include an abnormal anterior displacement of the posterior dura wall in flexion that resulted in an anteroposterior compression of the spinal cord segment from C7 to C8. To identify an anatomical relationship to this disorder, the lower cervical spine was studied using 11 embalmed adult human cadavers. Abundant posterior epidural ligaments were observed between the posterior dura mater and the ligamentum flavum. Posterior epidural ligaments in the cervical spine have not been reported previously. The anterior displacement of the dura mater may be attributed to a lack of and/or insufficiency of the posterior epidural ligaments.

In vivo macrophage recruitment by murine intervertebral disc cells.

SUMMARY An in vivo murine experiment was conducted to measure the capacities of viable intervertebral disc cells to recruit inflammatory cells. The objective was to determine whether compounds secreted from viable cells induce inflammation or whether inflammation in disc herniation simply requires exposure to structural cell or matrix components. Three tissue preparations were inserted into the right lower peritoneal cavity of male mice: tissue with viable annulus fibrosus and nucleus pulposus cells, tissue with viable annulus fibrosus cells, or devitalized annulus fibrosus and nucleus pulposus tissue. Controls included sham-operated and nonoperated groups. Mice were killed 1, 2, or 7 days after surgery. Macrophage recruitment occurred after exposure to viable disc tissue but not after exposure to devitalized disc components; recruitment increased over time. Viable disc cells play a role in the etiology of inflammation in disc herniation.

Growth factor modulation of distraction osteogenesis in a segmental defect model.

A model was established in 39 dogs to investigate the growth factor modulation of regenerate bone in distraction osteogenesis. A segment of the diaphysis of the radius was resected unilaterally. An osteotomy was made proximal to the segmental defect to create a transport segment. A monolateral external fixator was applied. After a latency period, the segment was transported across the defect. One week after the transport assembly contacted the distal pin clamp, an ipsilateral osteotomy of the proximal ulna was performed. In 20 dogs, transforming growth factor-beta was injected into the regenerate bone halfway through the transport period. Four dogs were sacrificed before docking, when the regenerate bone was still immature. In specimens harvested halfway through the transport period, evidence was found of intramembranous ossification during distraction. In specimens harvested after the transport assembly contacted the distal pin clamp, evidence was found that the mature regenerate formed by endochondral ossification. Therefore, a combined mechanism of ossification is proposed for this segmental defect model that includes mechanical stimulus for bone differentiation. The one-time administration of transforming growth factor-beta retarded the formation of a stable, united regenerate. It is concluded that transforming growth factor-beta caused an effect opposite to that which was desired.